Process for coating ceramic molds

ABSTRACT

The walls of the pattern cavity of a fired ceramic mold for use in metal casting are coated with a dispersion in a liquid medium of a particulate solid selected from graphite, molybdenum disulphide and aluminum and the liquid medium is evaporated to leave a deposit of the particulate solid of such thickness that the dimensions of the pattern cavity are not significantly altered. The coating reduces or eliminates mold-metal interaction thereby facilitating separating of the mould from the casting.

United States Patent 7 Taylor 51 Feb. 29, 1972 PROCESS FOR COATING CERAMIC MOLDS Inventor: Percy Ronald Taylor, Wrexham, Wales,

England Monsanto Chemicals Limited, London, England Feb. 12, 1970 Assignee:

Filed:

Appl. N0.:

Foreign Application Priority Data Feb. 19, 1969 Great Britain ..9,061/69 U.S.Cl ..117/5.1, 106/38.27,106/38.28, 117/160 A, 117/160 R, l 17/169 R, 164/72, 164/73 Int. Cl ..B44d l/20, B29c 1/04 Field ofSearch ..ll7/5.1, 169 R; 106/3827, 106/3828; 164/72, 73

References Cited UNITED STATES PATENTS 5/1962 Campbell ..1 17/51 X 2/1920 McKnight... ..106/38.27 X Greatrex ..117/5.1 X

Primary Examiner-William D. Martin Assistant Examiner-Mathew R. P. Perrone, Jr. Att0meyl-lerbert B. Roberts, Roger R. Jones and Neal E. Willis ABSTRACT The walls of the pattern cavity of a fired ceramic mold for use in metal casting are coated with a dispersion in a liquid medium of a particulate solid selected from graphite, molybdenum disulphide and aluminum and the liquid medium is evaporated to leave a deposit of the particulate solid of such thickness that the dimensions of the pattern cavity are not significantly altered. The coating reduces or eliminates mold-metal interaction thereby facilitating separating of the mould from the cast- 1 Claim, No Drawings PROCESS FOR COATING CERAMIC MOLDS This invention relates to improved ceramic moulds for metal casting and a process for the production of such moulds.

it is known that castings of metals and alloys when made by normal air casting techniques in hot ceramic moulds are liable to suffer from various surface defects. This is particularly true when large weights of metal are being poured into a mould, for instance in the production of large castings or castings with massive sections. Penetration defects tend to increase with higher metallostatic pressure. Surface defects are also likely to be caused by the adverse effects of wetting of the mould and reaction with metallic oxides produced from the metal cast.

Highly reactive metals such as for example aluminium, titanium or magnesium-base alloys, and other ferrous and nonferrous metals and alloys, are all susceptible to these surface defects. The defects are particularly evident in precision mould casting of the lost-wax type where smooth cast surfaces, close tolerances and easy detachment of the mould facing are required.

We have now discovered that these difiiculties can in many cases be overcome by using ceramic moulds having a pattern cavity wall surface modified according to the process of the invention.

The process of the invention is one in which the walls of the pattern cavity of a fired refractory ceramic mould are coated with a dispersion in a liquid medium of a particulate solid and the liquid medium is evaporated to leave a deposit of the particulate solid of such thickness that the dimensions of .the pattern cavity are not significantly altered, the particulate solid being characterized by having poor wetting properties relative to the metal or alloy or to the oxidation products of the metal or alloy to be cast in the mould, or by having an interleaved flakelike structure.

After deposition of the particulate solid, examples of which are graphite and aluminum, the mould is preferably refired for a short period before the metal is cast.

There is also provided by the present invention a mould having on the walls of the pattern cavity a coating comprising particulate aluminium.

The mould that is treated according to the process of the invention can be for example a shell, block or piece mould, but the improvements obtained by such treatment are particularly marked in the case of ceramic shell moulds. It is envisaged that the mould itself will have been made in conventional manner using an expendable or nonexpandable pattern and a particulate refractory material in conjunction with a binding agent such as a silica sol or a hydrolyzed ethyl silicate.

Examples of particulate solids characterized by poor wetting properties relative to metals and which are especially suitable for use in the present invention are graphite and molybdenum disulphide. Both these materials are commercially available as dispersions in which the particle size of the solid is microns or less, typically about 1 micron. The dispersion medium is usually an organic liquid, for example a ketone, ester or hydrocarbon, and such dispersions are particularly suitable for use in the present invention. Examples of solids having an interleaved, flakelike structure are aluminum and certain other metals in the finely divided form in which they are used for the production of aluminum and other metallic paints. The particle size of the solid is generally less than 45 microns. A metallic paint formulation, possibly thinned with a volatile organic solvent, can be used to coat the mould wall in accordance with the present invention.

Dispersions containing more than one type of solid, for example graphite and aluminum, can be used.

To improve the bonding of the coating to the mould wall it is possible to include in the dispersion a proportion of a hydrolyzed alkyl silicate binding agent of the type conventionally used in the formation of the main body of the mould.

The dispersion is usually applied by pouring it into the mould and subsequently decanting as much of the liquid as possible leaving only a very thin but continuous layer of added solid matter on the mould cavity surface. The thickness of the layer of deposited solid required to achieve the object of the invention will, in general, have no practical effect on the dimensions of the cavity, although the acceptable limits will vary according to the thickness of section and the dimensional tolerances in the casting. Typically, the thickness of the coating will not exceed microns, and thicknesses of for example from 10 to 50 microns are usually adequate. At low application rates, particles of colloidal graphite and molybdenum disulphide merely occupy pores in the wall of the pattern cavity, so that the dimensions of the pattern cavity are substantially unaltered.

After coating, the mould is preferably heated, for example by firing at a temperature in the range of from 600 to 1,1 50 C. The mould may be cooled to a lower temperature before the molten metal or alloy is poured into it. The optimum duration, temperature, and conditions of firing depend on the nature of the particulate solid. Graphite and molybdenum sulphide, for example, are oxidized fairly readily, and prolonged exposure to an oxidizing atmosphere at a high temperature is therefore to be avoided. For these materials firing at temperature in the range 600 to 700 C. for not more than 10 minutes is recommended where an ordinary furnace with an oxidizing atmosphere is used. Higher temperatures can be used where a furnace with a reducing atmosphere is available. 7

Aluminum is less susceptible to major change on heating than the materials mentioned above, and refiring of the mould after coating can therefore be carried out at higher temperatures. A coating of particulate aluminum is thus especially suitable where a metal or alloy of relatively high melting point, for example a steel, is to be cast into the mould, because it is generally desirable that the temperature of the metal should not greatly exceed that of the mould.

' When the metal or alloy has solidified and cooled, the mould may easily be detached from the casting.

The invention is illustrated by the following examples.

EXAMPLE I This example illustrates the method of the invention and the use of the treated mould in casting an aluminum alloy.

A ceramic shell mould was tired and cooled to room temperature. A dispersion of colloidal graphite in methyl ethyl ketone was poured into the mould cavity, and after ensuring that the cavity walls were completely coated, the excess dispersion was decanted.

The mould was then refired for 5 minutes at a temperature of 600 C. and allowed to cool to a cavity temperature of about 400 C. Molten L.M.20 aluminum was poured into the mould and allowed to cool.

On fracturing, the mould separated readily and cleanly from the metal, and the surface of the casting had an excellent finish.

In a comparative experiment, L.M.2O aluminum was cast under the same conditions into a mould which had not been treated with colloidal graphite but was otherwise identical. Separation of the mould from the casting was difi'rcult, with extensive adhesion of particles of mould material to the metal surface.

EXAMPLE 2 The procedure of Example 1 was repeated except that the dispersion used to coat the walls of the pattern cavity was an aluminum paint having good leafing properties, to which had been added 25 percent by volume of a solution in isopropanol of hydrolyzed ethyl silicate containing the equivalent of 5 percent by weight of silica. The coated mould was refired at 800 C. to give an aluminum film which was excellently bonded to the mould cavity surface.

What is claimed is:

1. A process for the production of an improved ceramic shell mould for metal casting, which comprises coating the walls of the pattern cavity of a fired ceramic shell mould with a dispersion in methylethyl ketone of particulate molybdenum disulphide having a particle size of microns or less and an hydrolyzed alkyl silicate, evaporating the methylethyl ketone to leave a deposit of the particulate molybdenum disulphide having a thickness of 10 to 50 microns, heating the mould to a temperature of 600 to 700 C. for not more than 10 minutes 5 to improve the bonding of the coating to the walls. 

